Pigments generally exhibit vivid color tone and high coloring power, and they are widely used in many fields. Examples of use applications in which pigments are used include paints, printing inks, electrophotographic toners, ink-jet inks, and color filters. In particular, examples of pigments that require high performance, and that are of particular importance in practical use, are ink-jet ink pigments, and color filter pigments.
In recent years, reduction in color filter thickness has been strongly required for achieving an increase in pixel count of apparatus associated with imaging, such as liquid crystal displays, CCD sensors or digital cameras. To reduction in color filter thickness, it is essential that finer pigments be used in color filters. In addition, development of pigment fine particles with uniformity and minuteness is required for ensuring higher contrast in color filters. In other words, development of pigment fine particles with minuteness, uniformity and stability holds the key to achieving high performance of apparatus associated with imaging.
On the other hand, dyes have been so far used as coloring materials of ink-jet inks. However, dyes are inferior in water resistance and light stability. So, pigments have come to be used for improvements in ink-jet ink properties. And it is being tried to apply ink-jet technology to not only a printing purpose but also production of a wide variety of precision members. For example, ink-jet technology is expected as a technology for production of precision members, most notably color filters, which substitutes for traditional technologies including lithography and allows enhancement of design flexibility and significant increase in productivity. However, neither pigment fine particles suitable for such a technology and fully adaptable to those requirements nor ink-jet inks containing such pigment fine particles are present yet.
From this background, pigments are required to be fined down so as to have particle diameters on the order of, for example, several tens of nanometers, and that to undergo such particle-diameter control that the distribution of their particle diameters approaches a monodisperse distribution. However, it is difficult to obtain such pigments by use of a general breakdown method (crushing method). This is because such a method requires great amounts of time and energy for crushing down pigments to nanometer-size particles, so it has low productivity, and besides, it limits pigments usable therein. In addition, it is known that, when too high energy is applied in the crushing method, an adverse effect referred to as overdispersion, such as a thickening phenomenon by re-aggregation, is caused.
Contrary to this, a build-up method in which particles are made to grow in a gas phase or a liquid phase has been studied. For example, methods of forming organic compound particles in a micro-chemical process are disclosed, and those methods make it possible to obtain fine particles with efficiency.
Although particles fined down so as to have diameters of several tens of nanometers have advantages in transparency, coloring power and the like, they suffer reduction in dispersion stability because of an increase in their specific surface areas (see Yuki Ganryo Handbook (Handbook of Organic Pigments), edited by Color Office, page 45). By contrast, there is a proposal to form fine particles encapsulated in a polymer by subjecting a polymerizable compound and a fine particle pigment dispersion containing a polymerization initiator to polymerization reaction by heating in a flask (see JP-A-2004-43776, wherein “JP-A” means unexamined published Japanese patent application). According to this method, however, there is an apprehension that variations in yield and molecular weight of the polymer produced by polymerization reaction lead to variations in performance, and the radical polymerization adopted exclusively in Examples is vulnerable to oxygen. In particular, it is thought that the method of using a flask (referred to as a batch method) is responsible for a rise in cost when mass production is carried out, and besides, temperature control therein is difficult, so fluctuations of temperature can become a cause of variations in quality. In other words, it has been desired to further develop production methods for dispersions of organic nanoparticles, such as nanoparticles of an organic pigment, which can ensure higher dispersion stability.
On the other hand, microchemical processes have an advantage in that they allow exacting temperature control of channels, and the performance of reactions or the like by flowing solutions through heated channels has been examined. For instance, it is reported by JP-A-2002-30230 that a pigment precursor is introduced into a microreactor for the purpose of conditioning an organic pigment and subjected to heat treatment, thereby yielding the pigment with an excellent hue. Although the document cited describes feeding of the pigment-in-organic solvent suspension into a microreactor, it neither aims to impart dispersion stability for keeping a volume-average particle diameter and particle size distribution of an aqueous dispersion of organic pigment fine particles nor contains any mention thereof. That document describes, e.g., in Example that, when a suspension of organic pigment in N-methylpyrrolidone is conditioned by heating at 180° C., the average particle diameter is enlarged from less than 70 nm to 221 nm. In other words, improvement of dispersion stability by heating is not a general phenomenon, if anything, heating is unsuitable for retention of a volume-average particle diameter because it causes an increase in viscosity and coarsening of particles.
Further, the method of preparing an emulsified dispersion from an oily solution containing a microcapsule wall material and an aqueous solution and subjecting the dispersion to microencapsulation by use of a microreactor is disclosed (JP-A-2002-282678, JP-A-2002-282679). However, the microcapsules prepared by such a method have sizes on the order of micrometers, and besides, they are unsatisfactory in both monodisperse property and dispersion stability.
In addition, the method of using a block copolymer and the method of utilizing a fluid in a supercritical or subcritical state (JP-A-2006-104448, JP-A-2006-124556) are known with respect to the production of fine particles of an organic pigment on the basis of a build-up process. Although these microchemical processes allow preparation of fine pigment dispersions, production conditions thereof are sharply restricted and they are lacking in general versatility.
Furthermore, there is a disclosure (JP-A-2007-39643) of the production method for an organic pigment dispersion liquid, wherein a polymerizable compound is incorporated into at least either an alkaline or acid solution containing an organic pigment in a dissolved state, or an aqueous medium, the pigment is precipitated in the form of fine particles under the process of mixing the solution and the aqueous medium, and then the polymerizable compound is made to polymerize; as a result, the polymer formed is fixed to the fine particles of pigment. Although preparation of a dispersion of superfine particles having good dispersion stability is tried in such a way, production methods capable of sufficiently ensuring satisfactory characteristics as leading-edge industrial materials such as color filters and ink-jet inks, and that at low cost and with stability, are not found yet, so further developments are desired.